1. Field of the Invention
The present invention relates to an optical recording medium for high-density information recording by photothermal converting effect, and a recording method utilizing said recording medium.
2. Related Background Art
A photothermal conversion recording medium employed as an optical disk, an optical card or the like can record information at a high density in the form of circular or spiral tracks of optically detectable small spots, for example 1 microns in size, formed in a thin photothermal conversion recording layer provided on a substrate. The information recording on such disk is achieved by scanning the surface of the recording layer with a focused laser beam to form pits, only in the surface irradiated with said laser beam, in the form of spiral or circular tracks. The recording layer is capable of forming optically detectable spots by absorbing the laser energy. For example, in the heat mode recording method, a laser sensitive layer absorbs the laser energy irradiated thereon to convert said energy into thermal energy thereby forming a small pit by evaporation or deformation, or a small spot, which is optically detectable by the difference in oxidation, reflectance or density, through a chemical reaction.
The information thus recorded is detected by scanning the track with a laser beam to read the optical changes between the pit portions and the pit-free portions. As an example, the track is scanned with a laser beam, and the energy reflected by the recording medium is monitored by a photodetector. Said photodetector provides a low output when the pit is not formed, but provides a high output when the pit is formed because the laser beam is more strongly reflected by the underlying substrate.
For such recording medium there have been principally proposed inorganic substances, for example thin metal layers such as evaporated aluminum layer, a thin bismuth layer, a thin tellurium oxide layer or amorphous chalcogenide glass layers. However such thin layers are generally not obtained by coating but have to be prepared by sputtering or vacuum evaporation, and, for this reason, they are expensive. In addition they are associated with other drawbacks such as a high reflectance to the laser beam, a high heat conductivity, and a low efficiency of laser beam utilization.
Recently compact, inexpensive and directly modulatable semiconductor lasers have been developed as laser light source, but such semiconductor lasers generally have an oscillation wavelength longer than 700 nm, and are generally weaker in power than gas laser such as argon laser or helium-neon laser. Thus, for achieving photothermal conversion recording with such semiconductor laser, the laser sensitive layer should preferably has an absorption peak at a long wavelength, generally in a region from 700 to 850 nm.
However, conventional photothermal conversion recording media do not have sufficient efficiency for converting the laser light energy into thermal energy. For example, in case of an optical disk, the photothermal conversion recording layer principally composed of the aforementioned inorganic substances cannot show a high sensitivity because such layer has a high reflectance to the laser beam and cannot effectively utilize the laser beam. Also a sensitive wavelength range in excess of 700 nm will complicate the layer structure of the laser sensitive layer. For these reasons there have been developed organic compounds capable of showing a change in response to the optical energy of a relatively long wavelength. There are already known compounds sensitive to the laser beam of a wavelength in excess of 700 nm, such as pyrylium compounds disclosed in the U.S. Pat. No. 4,315,983 or in the Research Disclosure 20517 (1981.5), and squalium dyes disclosed in J. Vac. Sci. Technol. 18(1), Jan./Feb. 1981, p. 105-109.
However, an organic compound having an absorption at a long wavelength generally requires a sensitivity at least of 100 mJ/cm.sup.2 for a laser beam of a wavelength of 830 nm in order to obtain a sufficiently high S/N ratio, and a higher powered laser beam is required for achieving a high speed scan recording in order to increase the transfer rate of the data. Thus the sensitivity of the laser sensitive laser should be improved in consideration of the service life of the hardware. However there have not been developed organic compounds of a sensitivity sufficient for such purpose.